Office chairs having tiltable chair backs are known. Such chair backs may utilize a spring to maintain positive supporting pressure against the back of a user. However, such chairs may present a hazard to users due to sudden movement of the chair back either under the force of the spring, or due to spring failure. Such hazards are undesirable and inconvenient.
Also known in the art are devices for arresting relative movement between a first part and a second. Such devices consist of a tubular housing fastened to the first part, a nut rotatably mounted in the housing and a spindle connected to the second part. The spindle passes through the housing, and the nut turns on the spindle, inside the housing, without self-locking. The nut, which has a conical shape, when translated axially with respect to the spindle movement, is brought into increasing frictional engagement with a conical stop sector at the inside wall of the housing. Such a device is described in prior art reference DE-A-39 07 251.
Reference DE-A-39 07 251 shows a jerk arresting device shown in FIG. 6. A nut 11' with conical braking surface 24' includes an internally threaded opening which causes nut 11' to rotate threadably on a shaft 4'. Shaft 4' is thrust into and out of a cylindrical housing 2' during a jerking motion of one part fixed to the shaft 4' with respect to another fixed to the cylindrical housing 2'. The jerking motion causes conical surface 24' to frictionally engage internal conical surface 9' fixed to cylindrical housing 2'. The prior art device of FIG. 6 does not provide positive engagement between the nut 11' and the cylindrical housing 2'. Since it relies on frictional engagement it is not reliable for its intended purposes. The same force that axially translates the rotating nut also provides the force for holding the braking surfaces 9' and 24' in frictional contact. The frictional contact must be disengaged in order for the chair to move as intended. This creates an additional force that must be overcome for there to be free movement of the chair back relative to its support.